The invention relates to minimizing inaccuracies in two-beam interferometer measurements that are due to vibration, temperature variations, air turbulence, nonlinearities in the phase of the reference beam, and errors in the starting points of a phase shifting mirror of the interferometer.
Use of phase shifting interferometry to make optical profilers or profilometers to measure the roughness of a test surface is known, for example, as described in "An Optical Profilometer for Surface Characterization of Magnetic Media", presented by Wyant et al. at the 38th Annual Meeting of the ASLE (American Society of Lubrication Engineers) in Houston, Tex., Apr. 24-28, 1983 and incorporated herein by reference, and also as described in "Optical Profilers for Surface Roughness", by Wyant, published in the Proceedings of the International Society for Optical Engineers, Vol. 525, Jan. 21-22, 1985, a copy of which is attached hereto as Appendix A. One phase shifting technique provides more effective and accurate height measurements than can be obtained by viewing interference fringes and measuring how far they depart from being straight and equally spaced, is the "phase stepping" technique, described in the foregoing references. Another phase shifting technique, first described by Wyant in "Use of an AC Hetrodyne Lateral Shear Inteferometer with Real-Time Wavefront Correction Systems", Applied Optics, Volume 14, No. 11, November 1975, page 2622, and incorporated herein by reference, is known as the integrating-bucket technique, wherein the reference mirror of the interferometer is moved at a constant velocity, rather than a stepped velocity. The integrating-bucket technique is often preferred, since less vibration is introduced into the system than when the movement of the interferometer mirror is stepped.
Those skilled in the art will recognize that any vibration of interferometry apparatus results in measurement inaccuracies. For example, in an optical profiler in which measurement techniques that approach the limits of the present state of the art, variations in air turbulence, and expansion and contraction of the apparatus as a result of changes in temperature, nonlinearities and calculation in the piezoelectric transducer and environmental effects on the piezoelectric transducer also are sources of significant error in the phase calculations and hence in the height measurements that must be made. As another example, in a laser diode tester using interferometry techniques to measure an interference pattern by means of a photodetector array and utilizing integrated bucket techniques to obtain the needed phase computations, the foregoing variations also are sources of significant error.
One approach for reducing such inaccuracies has been to use the single pass, four-bucket integrating technique, wherein all four "buckets" are utilized in a single calculation to obtain the phase for each detected point across the inteference pattern, as described by Wyant in the above November, 1975 article. This technique suffers from certain shortcomings, the main one being that it does not result in effective cancellation of sinusoidal error caused by phase differences other than 90.degree. between the integrated buckets, so severe errors are introduced phase computations by slight (i.e., half degree) errors in the 90.degree. phase shifts that constitute the integrating boundaries of the integrated buckets. Another prior approach has been to make a second pass and "collect" four more integrated buckets and use them to compute a second phase value, with the reference beam phase 90.degree. different than for the first phase value, and then average the first and second phase values. This technique, which is described in "Digital Wave-Front Measuring Interferometer: Some Systematic Error Sources" by Schwider, Applied Optics, November 1983, Volume 22, No. 21, page 3421, does not avoid errors caused by vibration and the other above-mentioned sources of error.
As those skilled in the art realize, there are numerous subtleties in the physics of interferometry. Slight alterations in the structure of the apparatus used and/or in the method of operating the apparatus may result in unexplained errors and/or anomalous results.
Thus, despite a strong market demand for faster, more accurate optical profilers, and despite extensive continuing research in the art, there still remains an unmet need for a reasonably priced optical profilometer (and other interferometry-based apparatus) that avoid the above-indicated sources of error more effectively than the prior art.